1. Field of the Invention
The subject invention generally relates to pressure transducers of the type employing a pressure-responsive diaphragm with silicon piezoresistive strain gauges epitaxially deposited thereon and, more particularly, to transducers of this type which employ integral digital temperature compensation for variation of the resistances and piezoresistivity of the strain gauges as a function of temperature.
2. Description of the Prior Art
Pressure transducers are known which are constructed by epitaxially depositing silicon piezoresistive strain gauges on a pressure responsive diaphragm. The piezoresistive strain gauges are generally, although not always, electrically connected in a bridge circuit such as a Wheatstone bridge. The voltage unbalance sensed across the bridge circuit is a measure of the pressure induced deflection of the diaphragm on which the strain gauges are deposited. The piezoresistive strain gauges are, however, subject to variations in resistance values and piezoresistivity characteristics due to changes in temperature for which compensation must be made in order to provide precise and accurate pressure measurements.
Temperature compensation has been provided in prior art silicon pressure transducers. For example, U.S. Pat. No. 3,646,815 to Martin et al discloses a transducer circuit including an L-type resistance bridge formed of a piezoresistor and a second resistor diffused into the diaphragm. The second transducer is insensitive to pressure and is connected to a temperature compensation circuit that provides outputs used to rebalance an amplifier and thereby make the measure of pressure insensitive to any changes in resistance values due to changes in temperature.
Other examples of temperature compensation of strain gauge pressure transducers are shown, for example, in U.S. Pat. No. 3,841,150 to Pearson and U.S. Pat. No. 4,196,382 to Bryzek. Both of these patents and the Martin et al. patent may be characterized as showing pressure transducers using exclusively analog circuitry. Since the measurement of such quantities as pressure, temperature and the like generally require a continuous output, the use of analog circuitry has been the obvious choice for transducers which measure such quantities. However, digital integrated circuits offer several advantages over traditional analog circuitry. These include miniaturization, lower cost, lower power requirements, and greater insensitivity to environmental conditions. Because of the discrete nature of digital signals, they are not directly usable in most transducer applications. In order to take advantage of the superior characteristics of digital circuitry, circuits have been developed which comprise a combination of analog and digital circuitry in an effort to maximize performance and minimize cost.
Examples of pressure transducers using a combination of analog and digital circuitry are shown, for example, in U.S. Pat. No. 4,321,832 to Runyan, U.S. Pat. No. 4,446,527 to Runyan, U.S. Pat. No. 4,592,002 to Bozarth, Jr., et al., and U.S. Pat. No. 4,598,381 to Cucci. The two patents to Runyan show a pressure transducer connected to analog signal processing circuitry. The output of the analog signal processing circuitry is connected to an analog-to-digital (A/D) converter, and both the analog signal processing circuitry and the A/D converter are controlled by a digital computer. Bozarth, Jr., et al. disclose a pressure transducer which provides its signal directly to an A/D converter the output of which is supplied to a digital computer. In this case, the signal processing is by the digital computer and not analog circuitry, and the digital signal processing includes compensation for temperature variations. Cucci discloses a differential pressure sensor where, again, signal processing is by a digital computer.
U.S. Pat. No. 4,320,664 to Rehn et al. discloses a thermally compensated silicon pressure sensor which, as best shown in FIG. 7, employs an A/D converter for supplying data to a microprocessor. This data includes temperature data as well as pressure data. Rehn et al. employ a read only memory (ROM) which stores compensation data. The Rehn et al. patent provides a good explanation of the need for thermal correction of a silicon strain gauge for the piezoresistivity parameter. U.S. Pat. No. 4,226,125 to Waugh also discloses a pressure sensor system which uses a microprocessor and a ROM. However, the use of a digital computer or a microprocessor is generally not justified for most transducer applications and adds considerably to the complexity of the system requiring software as well as hardware design.
Examples of pressure sensors employing a combination of analog and digital circuitry but not a digital computer or microprocessor are shown in U.S. Pat. No. 4,322,977 to Sell et al., U.S. Pat. No. 4,449,409 to Antonazzi and U.S. Pat. No. 4,532,809 to Antonazzi et al. Each of these patents discloses pressure transducers of the capacitive type as opposed to piezoresistive strain gauges; however, the signal processing circuitry of these patents include the use of analog-to-digital (A/D) and digital-to-analog (D/A) converters and temperature compensation of the measured pressure signal.
While the approaches taken in the prior art have generally been successful in terms of providing relatively precise and accurate temperature compensated pressure measurements, there is still a need for a simpler, more cost effective pressure transducer.